Molecular analysis of the xylFGH operon, coding for xylose ABC transport, in Thermoanaerobacter ethanolicus

A xylose ABC (ATP-binding cassette) transport operon, xylFGH, was cloned from Thermoanaerobacter ethanolicus, a thermophilic ethanol-producing eubacterium. The cistrons code for a periplasmic D-xylose-binding protein (XylF, partial sequence of 250 amino acids), ATP-binding protein (XylG, 505 amino acids), and integral membrane protein (XylH, 388 amino acids). These results, together with previous work, indicate that duplicate copies of both xylF and xylH are present in the T. ethanolicus chromosome, suggesting ancient gene duplication or lateral gene transfer events. XylG resembles other eubacterial monosaccharide ABC-ATPases in that its two nucleotide-binding domains (NBDs) are highly homologous, yet significantly different with respect to putative catalytic residues. Unlike most other integral membrane ABC transport proteins, XylH apparently contains 11 or 12 transmembrane segments (TMS) and is similar to a small group of ABC permeases that defy the "2 x 6" helix paradigm. This is the first report of a monosaccharide ABC transport operon in a thermophilic anaerobic eubacterium.